This invention relates to a signal detecting circuit for use with a pickup for sensing magnetized bars of ink moved past the pickup, and deals more particularly with such a detecting circuit capable of correctly identifying bar generated features in the output voltage waveform of the pickup despite wide differences in the magnetic strength of the individual bars and in the strength of the corresponding signals generated in the pickup by such bars.
The magnetic signal detector of this invention may be used in any of a wide variety of applications wherein information is conveyed by a plurality of spaced parallel bars or lines of magnetic ink printed on a suitable carrier and wherein the information is retrieved or read by magnetizing the ink and then moving the carrier past a pickup which for each bar or line moved therepast has a characteristic output voltage pattern or signature consisting of a first main pulse of one polarity followed by a second main pulse of opposite polarity. By way of example, the detector is shown herein in association with an envelope sorting apparatus and an encodable envelope insert such as disclosed in U.S. Pat. No. 3,895,220, which is incorporated herein by reference and to which reference may be made for further details of the sorter apparatus and of the encodable insert.
In the case of a sorting system, such as disclosed herein, the documents processed are often found to vary widely with regard to the quantity or density of the magnetic ink used to print the bars of the code. In one document the lay down of ink may be quite light so that signals of relatively small amplitudes are produced as the bars of that document are moved past the pickup. In another document the ink may be very heavy and produce signals of very large amplitude as the bars of that document pass the pickup. Further, in this latter situation the voltage signal generated in the pickup by each bar may also include an initial secondary pulse and a terminal or overshoot secondary pulse of significant amplitudes and either of which secondary pulses might be misinterpreted by existing prior art detectors as a pulse representing another bar of magnetic ink.
The general object of this invention is, therefore, to provide a magnetic signal detector for use with a magnetized bar sensing pickup which correctly identifies bars of magnetized ink passing the pickup despite wide variations in the lay down of the ink and particularly despite the fact that the ink may be so heavy as, for each bar, to produce in the output voltage of the pickup initial or terminal secondary pulses of substantial amplitude.